Method of manufacturing semiconductor article

ABSTRACT

A method of manufacturing a semiconductor article comprises steps of preparing a first substrate including a silicon substrate having a porous silicon layer and a nonporous semiconductor layer arranged on the porous silicon layer, bonding the first substrate and a second substrate to produce a multilayer structure with the nonporous semiconductor layer located inside, separating the first and second substrates of the multilayer structure from each other along the porous silicon layer by heating the multilayer structure and removing the porous silicon layer remaining on the separated second substrate.

This is a continuation of U.S. application Ser. No. 08/968,664 filed onNov. 12, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of manufacturing a semiconductorarticle that can suitably be used for producing a semiconductor devicesuch as a semiconductor integrated circuit, a solar cell, asemiconductor laser device or a light emitting diode. More particularly,it relates to a method of manufacturing a semiconductor articlecomprising a step of transferring a semiconductor layer onto asubstrate.

2. Related Background Art

Semiconductor articles are popular in terms of semiconductor wafers,semiconductor substrates and various semiconductor devices and includethose adapted for producing semiconductor devices by utilizing thesemiconductor region thereof and those used as preforms for producingsemiconductor devices.

Some semiconductor articles of the type under consideration comprise asemiconductor layer arranged on an insulator.

The technology of forming a single crystal silicon semiconductor layeron an insulator is referred to as silicon on insulator (SOI) technology,which is widely known. Various research has been done to exploit theremarkable advantages of SOI that cannot be achieved by using the bulkSi substrates that are used for producing ordinary Si integratedcircuits. The advantages of the SOI technology include:

1. the ease of dielectric isolation that allows an enhanced degree ofintegration;

2. the excellent resistivity against radiation;

3. a reduced floating capacitance that allows a high device operationspeed;

4. the omission of the well forming step;

5. the effect of latch up prevention; and

6. the possibility of producing fully depleted field effect transistorsusing the thin film technology. The advantages of the SOI technology arethoroughly discussed in the Special Issue: "Single-crystal silicon onnon-single-crystal insulators"; edited by G. W. Cullen, Journal ofCrystal Growth, volume 63, No. 3, pp. 429-590 (1983).

In recent years, a number of reports have been published on the SOItechnology for providing substrates that can realize high speedoperation and low power consumption for MOSFETs (IEEE SOI conference1994). The process of manufacturing a semiconductor device can besignificantly shortened by using the SOI structure if compared with thecorresponding process of manufacturing a device on a bulk Si wafer,because of the implementation of a very simplified device isolationstep. Thus, the use of the SOI technology can provide a significant costreduction in manufacturing a semiconductor device, particularly in termsof the wafer cost and the process cost if viewed from the conventionaltechnology of manufacturing a MOSFET or an IC on a bulk Si substrate, tosay nothing of the remarkable performance of such a semiconductordevice.

Fully depleted MOSFETs are very promising for achieving high speedoperation and low power consumption if provided with improved drivepower. Generally speaking, the threshold voltage (Vth) of a MOSFET isdetermined as a function of the impurity concentration of its channelsection but, in the case of a fully depleted (FD) MOSFET, thecharacteristics of the depletion layer are influenced by the SOI filmthickness. Therefore, the SOI film thickness has to be rigorouslycontrolled in order to improve the yield of manufacturing LSIs.

Meanwhile, a device formed on a compound semiconductor shows aremarkable level of performance that cannot be expected from silicon,particularly in terms of high speed operation and light emission. Suchdevices are currently formed by means of epitaxial growth on a compoundsemiconductor substrate that may be made of GaAs or a similar compound.However, a compound semiconductor substrate is costly and mechanicallynot very strong, so that it is not adapted to produce a large wafer.

Thus, efforts have been made to form a compound substrate byhetero-epitaxial growth on a Si wafer that is inexpensive, mechanicallystrong and good for producing a large wafer.

Research on forming SOI substrates became significant in the 1970s.Initially, attention was paid to the technique of producing singlecrystal silicon by epitaxial growth on a sapphire substrate (SOS:silicon on sapphire), that of producing an SOI structure through fullisolation by porous oxidized silicon (FIPOS) and the oxygen ionimplantation technique. The FIPOS method comprises steps of forming anislanded N-type Si layer on a P-type single crystal Si substrate byproton/ion implantation (Imai et al., J. Crystal Growth, Vol. 63,547(1983)) or by epitaxial growth and patterning, transforming only theP-type Si substrate into a porous substrate by anodization in a HFsolution, shielding the Si islands from the surface, and then subjectingthe N-type Si islands to dielectric isolation by accelerated oxidation.This technique is, however, accompanied by a problem that the isolatedSi region is defined before the process of producing devices, thereforerestricting the freedom of device design.

The oxygen ion implantation method is also referred to as the SIMOXmethod, which was proposed by K. Izumi for the first time. With thistechnique, oxygen ions are implanted into a Si wafer to a concentrationlevel of 10¹⁷ to 10¹⁸ /cm² and then the latter is annealed at hightemperature of about 1,320° C. in an argon/oxygen atmosphere. As aresult, the implanted oxygen ions are chemically combined with Si atomsto produce a silicon oxide layer that is centered at a depthcorresponding to the projection range (Rp) of the implanted ions. Underthis condition, an upper portion of the Si oxide layer that is turnedinto an amorphous state by the oxygen ion implantation is recrystallizedto produce a single crystal Si layer. While the surface Si layer used toshow a defect rate as high as 10⁵ /cm², a recent technologicaldevelopment has made it possible to reduce the defect rate down to about10² /cm² by selecting a rate of oxygen implantation of about 4×10¹⁷/cm². However, the allowable range of energy infusion and that of ionimplantation are limited if the film quality of the Si oxide layer andthe crystallinity of the surface Si layer are to be held to respectivedesired levels and hence the film thickness of the surface Si layer andthat of the buried Si oxide (BOX; buried oxide) layer are allowed totake only limited values. In other words, a process of sacrificeoxidation or epitaxial growth is indispensable to realize a surface Silayer having a desired film thickness. Such a process, in turn, givesrise to a problem of uneven film thickness due to the intrinsic adverseeffect of the process.

There have been reports saying that SIMOX can produce defective Si oxideregions in the Si oxide layer that are referred to as pipes. One of thepossible causes of the phenomenon may be foreign objects such as dustintroduced into the layer at the time of ion implantation. The deviceproduced in a pipe region can show degraded characteristics due to theleak current between the active layer and the underlying substrate.

The SIMOX technique involves the use of a large volume of ions that isby far greater than the volume used in the ordinary semiconductorprocess and hence the ion implantation process may take a long time if aspecifically designed apparatus is used for it. Since the ionimplantation process is performed typically by means of raster scan ofan ion beam showing a predetermined flow rate of electric current or byspreading an ion beam, a long time may be required for processing alarge wafer. Additionally, when a large wafer is processed at hightemperature, the slip problem due to an uneven temperature distributionwithin the wafer can become very serious. Since the SIMOX processrequires the use of extraordinary high temperature that is as high as1,320° C., which is not observed in the ordinary Si semiconductorprocess, the problem of uneven temperature distribution will become moreserious if a large wafer has to be prepared unless a highly effectiveapparatus is not realized.

Beside the above-described known techniques of forming SOI, a techniqueof bonding a single crystal Si substrate to another single crystal Sisubstrate that has been thermally oxized to produce an SOI structure hasbeen proposed recently. This method requires the use of an active layerhaving an even thickness for the devices to be formed on it. Morespecifically, a single crystal Si substrate that is as thick as severalhundred micrometers has to be made as thin as several micrometers orless. Three techniques have been known for thinning a single crystal Silayer that include:

(1) polishing;

(2) local plasma etching; and

(3) selective etching.

It is difficult to achieve an even film thickness by means of thepolishing technique. Particularly, the mean deviation in the filmthickness can be as large as tens of several percent to make thetechnique unfeasible when the film is thinned to an order ofsub-micrometer. This problem will become more significant for wafershaving a large diameter.

The technique of local plasma etching is typically used in combinationwith that of polishing. More specifically, the film is thinned by meansof polishing to about 1 to 3 μm and the distribution of film thicknessis determined by observing the film thickness at a number of points.Then, the film is subjected to an etching operation where the film isscanned with plasma of SF₆ particles having a diameter of severalmillimeters, correcting the distribution of film thickness, until adesired film thickness is obtained. There has been a report that thedistribution of film thickness can be confined within about ±10 nm orless by means of this technique. However, this process is accompanied bya drawback that, if foreign objects are present on the substrate in theform of particles during the plasma etching, they operate as etchingmasks to produce projections on the substrate when the etching operationis over.

Additionally, since the substrate shows a coarse surface immediatelyafter the etching operation, a touch-polishing operation has to beconducted on the surface after the end of the plasma etching and theoperation is controlled only in terms of its duration. Then, again theproblem of deviations in the film thickness due to polishing arises.Still additionally, a polishing agent typically containing colloidalsilica is used for the polishing operation and hence the layer formaking an active layer is directly scraped by the polishing agent sothat a crushed and/or distorted layer may be produced. The throughput ofthe process can be significantly reduced when large wafers are treatedbecause the duration of the plasma etching operation is prolonged as afunction of the surface area of the wafer being processed.

Selective etching involves the use of a film configuration for thesubstrate to be thinned that comprises one or more film layers adaptedto selective etching. For example, assume that a P⁺ -Si thin layercontaining boron by more than 10¹⁹ /cm³ and a P-type Si thin layer aremade to grow sequentially on a P-type substrate by means of epitaxialgrowth to produce a first substrate, which is then bonded to a secondsubstrate with an insulation layer interposed therebetween, theinsulation layer being typically an oxide film, and that the rearsurface of the first substrate is made sufficiently thin in advance byscraping and polishing. Subsequently, the P⁺ -layer is exposed byselectively etching the overlying P-type layer and then the P-typesubstrate is exposed by selectively etching the P⁺ -layer to produce anSOI structure. This technique is discussed in detail in a report byMaszara (W. P. Maszara, J. Electrochem. Soc., Vol. 138,341 (1991)).

While the selective etching technique is effective for producing a thinfilm with an even film thickness, it is accompanied by the drawbacks asidentified below.

The selective etching ratio is not satisfactory and will be as low as10² at most.

A touch-polishing operation is required to smooth the surface after theetching operation because of the coarse surface produced by the etchingoperation. Therefore, the film thickness can lose the uniformity as itis reduced by polishing. Particularly, while the polishing operation iscontrolled by the duration of the operation, it is difficult torigorously control the operation because the polishing rate can varysignificantly from time to time. Thus, this problem becomes significantwhen forming an extremely thin SOI layer that is as thin as 100 nm.

The produced SOI layer can show a poor crystallinity due to the use of afilm forming technique that involves ion implantation and epitaxial orhetero-epitaxial growth on a Si layer that is densely doped with B.Additionally, the bonded surface of the substrate may show a degree ofsmoothness that is inferior relative to that of a conventional Si wafer(C. Harendt, et al., J. Elect. Mater. Vol. 20,267 (1991), H. Baumgart,et al., Extended Abstract of ECS first International Symposium of WaferBonding, pp-733 (1991), C. E. Hunt, Extended Abstract of ECS firstInternational Symposium of Wafer Bonding, pp-696 (1991)). Stilladditionally, there is a problem that the selectivity of the selectiveetching technique heavily depends on the concentration difference amongthe impurities such as boron contained in the substrate and thesteepness of the concentration profile of the impurities along the depthof the substrate. Therefore, if the bonding anneal is conducted at hightemperature to improve the bonding strength of the layers and theepitaxial growth is carried out also at high temperature to enhance thecrystallinity of the SOI layer, the concentration profile of theimpurities along the depth becomes flattened to reduce the selectivityof the etching operation. Simply stated, the improvement of the etchingselectivity and hence that of the crystallinity and the improvement ofthe bonding strength are conflicting requirements that cannot be met atthe same time.

Under these circumstances, the inventors of the present inventionproposed a novel method of manufacturing a semiconductor article inJapanese Patent Application Laid-Open No. 5-21338. According to theinvention, the proposed method is characterized by forming an article byarranging a nonporous single crystal semiconductor region on a poroussingle crystal semiconductor region, bonding the surface of a materialcarrying an insulating material thereon to the corresponding surface ofsaid porous single crystal semiconductor region and subsequentlyremoving said porous single crystal semiconductor region by etching.

T. Yonehara et al., who are the inventors of the present invention, alsoreported a bonded SOI that is excellent in terms of even film thicknessand crystallinity and adapted to batch processing (T. Yonehara et al.,Appl. Phys. Lett. Vol. 64,2108 (1994)). Now, the proposed method ofmanufacturing a bonded SOI will be summarily described below byreferring to FIGS. 3A through 3C of the accompanying drawings.

The proposed method uses a porous layer 32 formed on a first Sisubstrate 31 as a layer to be selectively etched. After forming anonporous single crystal Si layer 33 on the porous layer 32 by epitaxialgrowth, it is bonded to a second substrate 34 with a Si oxide layer 35interposed therebetween (FIG. 3A). Then, the porous Si layer is exposedover the entire surface area of the first substrate by scraping off thefirst substrate from the rear side (FIG. 3B). The exposed porous Si isthen etched out by means of a selective etching solution typicallycontaining KOH or HF+H₂ O₂ (FIG. 3C). Since the selective etching ratioof the operation of etching the porous Si layer relative to the bulk Silayer (nonporous single crystal Si layer) can be made as high ashundreds of thousands with this technique, the nonporous single crystalSi layer formed on the porous layer in advance can be transferred ontothe second substrate to produce a SOI substrate without reducing thethickness of the nonporous single crystal Si layer. Thus, the uniformityof the film thickness of the SOI substrate is determined during theepitaxial growth step. According to a report by Sato et al., since a CVDsystem adapted to an ordinary semiconductor process can be used for theepitaxial growth, a degree of uniformity of the film thickness as highas 100 nm±2% can be realized. Additionally, the epitaxial Si layer showsan excellent crystallinity of about 3.5×10² /cm².

Since the selectivity of any conventional selective etching techniqueheavily depends on the concentration difference among the impuritiescontained in the substrate and the steepness of the concentrationprofile of the impurities along the depth of the substrate as describedabove, the temperature of the heat treatment (for bonding, epitaxialgrowth, oxidation and so on) is limited to as low as 800° C. at mostbecause the impurity concentration profile becomes flattened above thattemperature limit. On the other hand, the etching rate of the proposedetching technique is mainly determined by the structural differencebetween the porous layer and the bulk layer so that the heat treatmentis not subjected to such a rigorous limitation and temperature as highas 1,180° C. can be used. It is known that a heat treatment processconducted after the bonding operation can remarkably improve the bondingstrength between wafers and reduce the size and number of voids on thebonding interface. Additionally, with a selective etching operationdepending the structural difference between the porous layer and thebulk layer, the uniformity of the film thickness is not adverselyaffected by fine particles that can be adhering to the porous Si layer.

However, a semiconductor substrate produced by a bonding processinevitably requires at least two wafers as starting materials, one ofwhich is substantially wasted away in the course of polishing andetching to consume the limited natural resources almost for nothing. Inother words, a SOI manufacturing process is required to realize low costand economic feasibility in addition to an enhanced degree of processcontrollability and an improved uniformity of the film thickness.

Differently stated, the requirements of a process for manufacturing ahigh quality SOI substrate include an excellent reproducibility, anenhanced level of resource saving capability through the repeated use ofa same wafer and low manufacturing cost.

Under these circumstances, the inventors of the present inventionproposed in Japanese Patent Application Laid-Open No. 7-302889 a methodof manufacturing a semiconductor substrate, with which a pair ofsubstrates are bonded together and subsequently separated from eachother through a porous layer arranged therebetween so that one of thesubstrates may be reused by removing the porous substance remaining onit. The disclosed method will now be summarily described below byreferring to FIGS. 4A through 4C of the accompanying drawings.

It comprises steps of forming a porous layer 42 by transforming asurface layer of a first Si substrate 41 into a porous state, forming asingle crystal Si layer 43 on the porous layer, bonding the singlecrystal Si layer to the main surface of a second Si substrate 44 with aninsulation layer 45 interposed therebetween (FIG. 4A). It furthercomprises steps of separating the wafers bonded together with the porouslayer arranged therebetween (FIG. 4B) and selectively removing theexposed porous Si layer on the surface of the second Si substrate toproduce a SOI substrate (FIG. 4C). With this method, the first substrate41 can be reused after removing the residual porous layer. The bondedwafers may be separated from each other typically by way of one of thefollowing techniques;

applying sufficiently strong tensile force or pressure onto a surface ofthe combined wafers along a direction perpendicular to the surface;

applying wave energy in the form of an ultrasonic wave or the like tothe combined wafers;

causing the porous layer to be exposed at an end surface of the combinedwafers, etching the porous Si layer to a certain extent and insertingthe edge of a blade;

causing the porous layer to be fully exposed at an end surface of thewafers, soaking the porous Si layer with liquid that may be water andcausing the liquid to expand by entirely heating or cooling the combinedwafers; and

applying force to the first (or second) substrate along a directionparallel to the second (or first) substrate in order to destroy theporous Si layer.

The above listed techniques are based on the idea that, while themechanical strength of the porous Si layer depends on the porosity ofthe layer, it is sufficiently lower than that of a bulk Si layer. As arule of thumb, a porous Si layer having a porosity of 50% shows amechanical strength about a half of that of a corresponding bulk Silayer. In short, when a pair of bonded wafers is subjected tocompressive, tensile or shearing force, the porous Si layer will bedestroyed to begin with. A porous layer showing a higher degree ofporosity can be destroyed with less force.

However, in reality, efforts have been paid to reduce the porosity ofthe surface layer of the porous Si in order to realize an excellentepitaxial growth in terms of the quality of the device formed on the SOIsubstrate, while increasing the porosity of the inside of the porous Sifor easy separation of the bonded wafers. Thus, as described in anexample disclosed in Japanese Patent Application Laid-Open No. 7-302889,it has been a known practice to modify the porosity of the porous Silayer by controlling the electric current used in an anodizationprocess.

On the other hand, Japanese Patent Application Laid-Open No. 8-213645discloses a method of mechanically destroying a porous Si layer in orderto separate a device forming layer from a substrate to which the formerhas been bonded, although it does not describe the configuration of theporous layer. Anyhow, conventionally, a pair of bonded substrates areseparated along a porous layer arranged therebetween either bymechanically destroying the porous layer or by controlling the electriccurrent used in an anodization process to modify the porosity of theporous layer.

Of these, the technique of applying external force to the bonded wafersto separate them along the porous layer disposed therebetween can resultin unintended separation of the wafers along the bonded surfaces thereofif the bonding strength holding the wafers together is smaller than themechanical strength of the porous Si layer or if the porous layer hasone or more than one mechanically weak local regions. If a techniquethat does not involve a bonding process is employed, the process ofseparating the wafers along the porous layer has to be controlledrigorously in order to separate them mechanically without fail.

Japanese Patent Application Laid-Open No. 5-211128 proposes a method ofseparating a pair of bonded wafers comprising a step of forming a bubblelayer by ion implantation and a subsequent step of crystal rearrangementand cohesion of bubbles by heat treatment so that the wafers may bepeeled off from each other along the bubble layer. However, this methodis accompanied by a problem of difficulty with which the heat treatmentis optimized and the use of a low temperature range between 400 and 600°C. It is not possible to suppress the above-described generation ofvoids with such a low temperature range, which voids cannot beeliminated if the bonded wafers are subjected to another heat treatmentprocess after the formation of a thin film. In other words, thereduction in the size and number of voids is a phenomenon that appearswhen the pair of bonded wafers are heat treated at high temperature andwould not occur if the bonded wafers are heat treated after theformation of a thin film. The net result of such an additional heattreatment will be an increased strength of the zone binding the waferstogether. Additionally, this method involves a step of polishing thesurfaces of the substrates after they are peeled off from each other,which step can degrade the distribution of film thickness.

As described above, each of the known techniques of separating asubstrate along a porous layer is accompanied by its specific problemsthat have to be dissolved to adapt itself to the rapidly expandingapplications of the bonded SOI technology, which will be summarilydescribed below.

A light transmitting substrate typically made of glass plays animportant role in a contact sensor comprising a light receiving deviceor a projection type liquid crystal image display apparatus. A highperformance drive device is required to realize a higher density, anenhanced resolution and an improved definition for the pixels arrangedin such a sensor or a display apparatus. To meet this requirement, it isnecessary to form a single crystal layer on a light transmittingsubstrate so that the devices arranged on the substrate may also show anexcellent crystallinity. Additionally, the use of such a single crystallayer makes it possible to implement a peripheral circuit for drivingpixels and a circuit for processing images on a substrate carrying thepixels on it in order to downsize the chip and increase its operatingspeed.

However, a light transmitting substrate typically of glass can carrythereon only a non-crystalline thin Si layer or a polycrystalline thinSi layer at best to reflect the disorganized crystal structure of thesubstrate and hence such a substrate is not adapted to high performancedevices. This is principally because the substrate shows anon-crystalline structure and hence cannot produce a high quality singlecrystal layer on it if a Si layer is formed thereon by deposition.

In other words, a non-crystalline Si layer or a polycrystalline Si layeris not adapted to produce a drive circuit on it that operatessatisfactorily because of its defective crystal structure. This is whythere is an ever-increasing demand for an advanced SOI technology forproducing SOI substrates including bonded SOI substrates.

Although the use of a compound semiconductor substrate is indispensablefor manufacturing a compound semiconductor device, compoundsemiconductor substrates are costly and mechanically not strong so thatthey are not adapted to producing large wafers. Therefore, efforts havebeen paid to produce a compound semiconductor by hetero-epitaxial growthon a Si wafer that can easily be made to have a large surface area.

While research is being made to epitaxially grow a compoundsemiconductor such as GaAs on a Si substrate, the grown film typicallyshows a poor crystallinity and hence is poorly adapted to being used forsemiconductor devices mainly due to the difference in the latticeconstant and the thermal expansion coefficient between them.

Meanwhile, research is also being made to epitaxially grow a compoundsemiconductor on a porous Si layer in order to mitigate the aboveidentified lattice misfit. However, a porous Si layer is thermallyunstable and can change with time so that it is not stable or reliableas a substrate during and after the operation of forming devicesthereon. Thus, there is a need for a technology of producing a bondedSOI substrate with which a compound semiconductor is made to epitaxiallygrow on a porous Si layer and the grown compound semiconductor istransferred onto another substrate.

SUMMARY OF THE INVENTION

In view of the above-described circumstances, it is therefore an objectof the present invention to provide a method of manufacturing asemiconductor article comprising a step of bonding a pair of substrates,wherein part of the substrates is reused as raw material formanufacturing another semiconductor article.

Another object of the present invention is to provide a method ofmanufacturing a semiconductor article, characterized by comprising stepsof preparing a first substrate including a silicon substrate having aporous silicon layer and a nonporous semiconductor layer arranged on theporous silicon layer, bonding the first substrate and a second substrateto produce a multilayer structure with the nonporous semiconductor layerlocated inside, separating the first and second substrates of themultilayer structure from each other along the porous silicon layer byheating the multilayer structure and removing the porous silicon layerremaining on the separated second substrate.

Still another object of the present invention is to provide a method ofmanufacturing a semiconductor article, characterized by preparing afirst substrate including a silicon substrate having a porous siliconlayer and a nonporous semiconductor layer arranged on the porous siliconlayer, bonding the first substrate and a second substrate to produce amultilayer structure with the nonporous semiconductor layer locatedinside, separating the first and second substrates of the multilayerstructure from each other along the porous silicon layer by heating themultilayer structure, removing the porous silicon layer remaining on theseparated second substrate and reusing the substrate obtained byremoving the porous layer remaining on the separated first substrate asmaterial of another first substrate.

A further object of the present invention is to provide a method ofmanufacturing a semiconductor article, characterized by preparing afirst substrate including a silicon substrate having a porous siliconlayer and a nonporous semiconductor layer arranged on the porous siliconlayer, bonding the first substrate and a second substrate to produce amultilayer structure with the nonporous semiconductor layer locatedinside, separating the first and second substrates of the multilayerstructure from each other along the porous silicon layer by heating themultilayer structure, removing the porous silicon layer remaining on theseparated second substrate and reusing the substrate obtained byremoving the porous layer remaining on the separated first substrate asmaterial for another second substrate.

With the known methods of separating the bonded substrates of amultilayer structure along the porous Si layer by externally applyingpressure to the structure, the substrates would separate from each otheralong the interface that is mechanically not strong or has, if any,mechanically weak areas. To the contrary, the method according to theinvention utilizes the fact that the porous Si layer is structurallyfragile and comprises a step of heating the entire multilayer structureobtained by bonding a pair of substrates or at least the porous Si layeror its vicinity so that the substrates can be separated from each otheralong the porous Si layer with ease because of the thermal stressgenerated there and/or the mollified porous Si layer. Therefore, theconfiguration of the porous Si layer does not provide any problem. Inother words, its porosity may be uniform or differentiated to stratifythe layer in terms of porosity. Additionally, according to theinvention, the porous Si layer can be subjected to internal pressurethat is attributable to the thermal stress generated there by utilizingthe fragility of porous Si so that the substrates may be separated alongthe porous Si layer in a well controlled manner.

With the known methods of manufacturing a substrate by bonding a pair ofcomponent substrates, the first substrate (Si substrate) is scraped oretched off gradually from a side thereof so that it is not allowed tobond the first substrate onto a support structure on either side. Withthe method of the present invention, to the contrary, the firstsubstrate is allowed to maintain its original profile throughout theentire process except a surface layer so that it may be bonded onto asupport structure on either side in such a way that a pair of multilayerstructure substrates may be prepared by using a single first substrateto remarkably improve the manufacturing productivity. It will beappreciated that, with this arrangement, the first substrate can bereused after it is separated from the second substrate.

Additionally, with the method of the present invention, the firstsubstrate can be separated from the second substrate by utilizing theporous layer that has a large area to reduce the time and cost requiredfor the separating step and efficiently transfer a nonporous thin filmthat is a single crystal Si layer or a single crystal compoundsemiconductor layer having a very large and flat surface area andshowing an excellent crystallinity. In other words, the method of thepresent invention provides a SOI structure comprising a single crystalSi layer formed on an insulation layer and having an even film thicknessat high yield.

If laser is used as heating means for the purpose of the invention, onlyone or more than one specific layers can be made to absorb laser energyand become heated without heating the entire substrate obtained bybonding component substrates. More specifically, by selecting a laserbeam having a wavelength that can be absorbed only by the porous Silayer or a layer located near the porous Si layer, the layer may belocally heated.

On the other hand, with the method of the present invention, the porousSi layer may be heated by causing an electric current to flow throughthe porous Si layer or along a plane located close to the porous Silayer.

According to the invention, the first substrate (Si substrate) can bereused after separating the porous Si layer substrate therefrom.Furthermore, this first substrate (Si substrate) may be reused for anumber of times until it no longer be used due to a reduced mechanicalstrength.

The method according to the invention is free from scraping, polishingand etching steps that are indispensable for the comparable knownmethods for removing the first substrate and exposing the porous Silayer, because the two component substrates can be separated at a timealong the porous layer having a large surface area according to theinvention. Additionally, the plane along which the two componentsubstrates are separated can be strictly defined to a given depth in theporous Si layer by implanting ions of at least one of a rare gaselement, hydrogen and nitrogen with a projection range corresponding tothe given depth in the porous Si layer so that the porous layer left onthe second substrate shows an even thickness and can be removeduniformly by means of an etching solution with minimal selectivity.

Thus, according to the invention, a nonporous semiconductor layer (an Silayer, a compound layer or some other layer) having a large surfacearea, a even thickness and an excellent crystallinity can be formedeconomically on a second substrate (made of a semiconductor, aninsulator or some other material).

Therefore, the present invention provides a method of manufacturing asemiconductor article comprising a nonporous semiconductor layer (whichmay be a single crystal Si layer or a single crystal compoundsemiconductor layer) formed on a transparent substrate (lighttransmitting substrate) and having a crystallinity comparable to asingle crystal wafer that is excellent in terms of productivity, eventhickness, controllability and cost.

Additionally, the present invention provides a semiconductor articlecomprising a single crystal semiconductor layer showing an evenly flatsurface and an excellent crystallinity over a large area by means of aselective etching technique that can realize a remarkably high selectiveetching ratio.

Finally, the present invention provides a method of manufacturing asemiconductor article that can be used in place of costly SOS or SIMOXfor preparing a large scale integrated circuit having a SOI structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C are schematic cross sectional lateral views of asemiconductor article being manufactured by a method according to theinvention, illustrating different manufacturing steps.

FIGS. 2A, 2B and 2C are schematic cross-sectional lateral views of asemiconductor article being manufactured by a method according to theinvention, illustrating different manufacturing steps in another mode ofcarrying out the invention.

FIGS. 3A, 3B and 3C are schematic cross sectional lateral views of asemiconductor article being manufactured by a first known method.

FIGS. 4A, 4B and 4C are schematic cross sectional lateral views of asemiconductor article being manufactured by a second known method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described in greater detail in termsof preferred modes and different phases of carrying out the invention.However, it will be appreciated that the present invention is by nomeans limited thereto and covers any other modes of realizing theinvention that can be used for the purpose of the invention.

Preparation of Porous Silicon

Porous Si was discovered in 1956 by Uhlir et al. who were studying aprocess of electropolishing a semiconductor material (A. Uhlir, BellSyst. Tech. J., Vol. 35,333 (1956)). Porous Si can be prepared throughanodization of a Si substrate in an HF solution. Unagami reports as aresult of his study on the dissolutive reaction of Si in a Sianodization process that the existence of holes is required foranodization of Si and the reaction proceeds in a manner as describedbelow (T. Unagami, J. Electrochem. Soc., Vol. 127,476 (1980)).

    Si30 2HF+(2-n)e.sup.+ →SiF.sub.2 +2H.sup.+ +ne.sup.-

    SiF.sub.2 +2HF→SiF.sub.4 +H.sub.2

    SiF.sub.4 +2HF→H.sub.2 SiF.sub.6

or

    Si+4HF+(4-λ)e.sup.+ →SiF.sub.4 +4H.sup.+ +λe.sup.-

    SiF.sub.4 +2HF→H.sub.2 SiF.sub.6

where e⁺ and e⁻ represent respectively a hole and an electron and n andk represent respective numbers of holes required for dissolving a singleSi atom. The report says that porous Si is formed when the condition ofn>2 or λ>4 is met.

Although a conclusion that can be drawn from the above is that P-type Sican be made porous under the existence of holes whereas N-type Si cannotbe made porous, in reality, both N-type Si and P-type Ni can be turnedporous under certain conditions.

According to the invention, single crystal porous Si can be formedthrough anodization of a single crystal Si substrate typically in a HFsolution. A porous Si layer shows a spongy structure where pores with adiameter between 10⁻¹ and 10 nm are arranged with intervals between 10⁻¹and 10 nm. The density of porous Si can be made to vary between 2.1 and0.6 g/cm³ by varying the concentration of the HF solution between 50 and20% and/or by varying the current density in contrast to the density ofsingle crystal Si that is equal to 2.33 g/cm³. In other words, theporosity of porous Si is variable. While porous Si can be made to show adensity less than a half of that of single crystal Si, it maintains theproperties as single crystal Si so that a single crystal Si layer can beformed by epitaxial growth on a porous Si layer.

A porous Si layer has a density that is less than the density of asingle crystal Si layer because it contains a large number of voids inthe inside. Consequently, a porous Si layer shows a dramatically largesurface area relative to the volume it occupies. This means that aporous Si layer can be etched at a rate by far greater than the rate atwhich an ordinary single crystal Si layer is normally etched.

While porous Si shows a mechanical strength that varies depending on itsporosity, it is presumably lower than that of bulk Si. For instance, ifa porous Si layer shows a porosity of 50%, it may be safe to assume thatits mechanical strength is about a half of that of a comparable bulk Silayer. In other words, when a wafer formed by bonding a pair ofsubstrates is subjected to compressive, tensile or shearing force, theporous Si layer arranged therebetween will be destroyed first. If thelayer has a large porosity, it will be destroyed with little effort.

There are reports saying that micro-cavities having a diameter betweenseveral nanometers and tens of several namometers can be formed in apiece of bulk Si to a concentration of 10¹⁶⁻¹⁷ /cm³ by implanting heliumor hydrogen ions and heat treating the piece particularly in the areawhere ions are implanted (see, inter alia, A. Van Veen, C. C. Griffioenand J. H. Evans, Mat. Res. Soc. Symp. Proc. 107 (1988, Material Res.Soc. Pittsburgh, Pa.) p. 449). Recently, research is being conducted forutilizing a group of micro-cavities for a gettering site of a metalimpurity.

In an experiment conducted by V. Raineri and S. U. Campisano, heliumions were implanted into a substrate of bulk Si, which was then heattreated to form a group of micro-cavities therein and subsequentlysubjected to an oxidation process where a groove was formed in thesubstrate to expose a lateral side of the micro-cavity group. Theyreport that the micro-cavity group was selectively oxidized to produce aburied Si oxide layer, which showed an SOI structure (V. Raineri and S.U. Campisano, Appl. Phys. Lett. 66 (1995) p. 3654). However, with thetechnique they employed, the thickness of the surface Si layer and thatof the buried Si oxide layer are limited to respective ranges that allowboth the formation of a micro-cavity group and relaxation of the stressgenerated by the increased volume at the time of oxidation and anoperation of forming a groove is necessary for selective oxidation sothat a SOI structure cannot be produced on the entire surface of thesubstrate.

Nonporous Semiconductor Layer

For the purpose of the present invention, a nonporous semiconductorlayer can be formed preferably by using a material selected from singlecrystal Si, polycrystalline Si, noncrystalline Si and compoundsemiconductors including GaAs, InP, GaAsP, GaAlAs, InAs, AlGaSb, InGaAs,ZnS, CdSe, CdTe and SiGe. A nonporous semiconductor layer that can beused for the purpose of the present invention may substantially containone or more than one FETs (field effect transistors).

First Substrate

For the purpose of the present invention, the first substrate includes asilicon substrate having therein a porous silicon layer and carrying anonporous semiconductor layer arranged on the porous silicon layer. Aninsulation layer such as SiN, SiO₂ may be formed on the nonporoussemiconductor layer. It may be prepared by forming a nonporoussemiconductor layer on the porous silicon layer in the silicon substrateor by forming a porous silicon layer in part of a silicon substratehaving therein a nonporous semiconductor layer.

A nonporous semiconductor layer can be formed on a porous silicon layertypically by means of a CVD technique selected from vacuum CVD, plasmaCVD, photo CVD and MO CVD (metal-organic CVD), a sputtering technique(including a bias sputtering technique), a molecular beam epitaxialgrowth technique or a liquid phase growth technique.

Second Substrate

For the purpose of the present invention, the second substrate ontowhich the nonporous semiconductor layer is transferred from the firstsubstrate may be selected from a semiconductor substrate such as asingle crystal silicon substrate, a semiconductor substrate carrying aninsulation film such as an oxide film (including a thermally oxidizedfilm) or a nitride film on the surface thereof, a light transmittingsubstrate such as a silica glass substrate or a glass substrate, a metalsubstrate and an insulating substrate typically made of aluminadepending on the application of the finally prepared semiconductorarticle.

Bonding

For the purpose of the invention, the first and second substrates arebonded to each other to form a multilayer structure with the nonporoussemiconductor layer located inside. The multilayer structure may containan insulation layer between the nonporous semiconductor layer and thesecond substrate. The first and second substrates can be firmly bondedtogether typically at room temperature by smoothing their bondingsurfaces. Additionally, techniques including anodic bonding,pressurization and thermal treatment may appropriately be used toimprove the bonding strength.

Heating of the Multilayer Structure

For the purpose of the invention, the multilayer structure obtained bybonding a first substrate having a porous silicon layer and a nonporoussemiconductor layer and a second substrate (in such a way that thenonporous semiconductor layer is located inside) is separated along theporous silicon layer in order to transfer the nonporous semiconductorlayer onto the second substrate by heating the multilayer structure. Inthe heating process, the entire multilayer structure may be heated or,alternatively, only a specific part of the multilayer structure such asthe porous silicon layer may selectively be heated. The specific heatingmeans that can be used for the purpose of the invention may be a furnace(e.g., a heat treatment furnace) for heating the multilayer structure toabout 600 to 1,200° C. or a laser irradiation apparatus for causing aspecific layer to absorb the irradiated laser energy and become heatedwithout heating the remaining areas of the multilayer structure. Such alaser apparatus may use a laser beam having a wavelength that isabsorbed only by the porous Si layer or a layer located close to theporous Si layer and hence adapted for local heating.

Another feasible local heating arrangement may be the use of an electriccurrent that is made to flow along the porous Si layer or a layerlocated close to the porous Si layer in order to heat the porous Silayer.

Removal of the Porous Layer

After separating the multilayer structure that has been prepared bybonding first and second substrates together along the porous Si layer,the residual porous Si remaining on the substrates can be selectivelyremoved on the basis that the porous Si layer has a low mechanicalstrength and a large surface area. Methods that can be used forselectively removing the remaining porous Si include mechanicaltechniques such as scraping and polishing, chemical etching using anetching solution and ion etching (such as reactive ion etching).

Etching solutions that can be used for a process of selectively removingthe porous Si by means of an etching solution include, beside a mixturesolution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide,hydrofluoric acid, a mixture solution obtained by adding alcohol tohydrofluoric acid, a mixture solution obtained by adding alcohol andaqueous hydrogen peroxide to hydrofluoric acid, buffered hydrofluoricacid, a mixture solution obtained by adding alcohol to bufferedhydrofluoric acid, a mixture solution obtained by adding aqueoushydrogen peroxide to buffered hydrofluoric acid, a mixture solutionobtained by adding alcohol and aqueous hydrogen peroxide to bufferedhydrofluoric acid and a mixture solution of hydrofluoric acid, nitricacid and acetic acid.

The semiconductor article having the transferred nonporous semiconductorlayer may preferably be heat treated in a hydrogen-containingatmosphere, subsequently to the selective removal of the porous layer,to improve the flatness of the nonporous semiconductor layer.

Now, the present invention will be described by referring to theaccompanying drawings that illustrate preferred modes of carrying outthe invention.

First Mode of Carrying out the Invention

In this mode of carrying out the invention, to begin with, at least asingle nonporous thin film 13 is formed on the principal surface of afirst single crystal Si substrate 11 with a porous Si layer 12 formedright under the nonporous thin film 13. The nonporous thin film 13 andthe porous Si layer 12 can be formed by any of the techniques as listedbelow:

a) forming a porous Si layer 12 by anodization and subsequently forminga nonporous thin film 13 on the porous Si layer;

b) forming a porous Si layer 12 and a nonporous thin film 13simultaneously by implanting ions of at least one of a rare gas element,hydrogen and nitrogen; and

c) implanting ions of at least one of a rare gas element, hydrogen andnitrogen in addition to the technique of a) above to form regions withdifferent porosities.

The nonporous thin film 13 may comprise single crystal Si,polycrystalline Si, noncrystalline Si, metal film, compoundsemiconductor thin film or superconductive thin film. It mayadditionally comprise a device structure containing one or more than onedevices such as MOSFETs. The surface level of the bonding interface canbe separated from the active layer preferably by additionally forming anuppermost SiO₂ layer. If the layer implanted with ions is observedthrough a transmission electron microscope, it will be found that thereare numerous micro-cavities. The charged state of ions being implanteddoes not matter for the purpose of the invention. The accelerationenergy applied to ions being implanted will be so controlled that theirprojection range matches the desired depth of ion implantation. Whilethe size and the concentration of the micro-cavities may vary dependingon the rate of ion implantation, ions are preferably implanted at a rategreater than about 1×10¹⁴ /cm² and preferably greater than 1×10¹⁵ /cm².For achieving a long projection range, a channeling ion implantationtechnique may be used. After the ion implantation, the multilayerstructure is subjected to a heat treatment process whenever necessary.Referring to FIG. 1A, a surface of the second substrate 14 and acorresponding surface of the first substrate are brought into closecontact at room temperature so that they may be bonded together.Subsequently, the bonding strength between the two substrates can beintensified by means of anode junctioning, pressurization, heattreatment, if appropriate, or a combination of any of these.

When a single crystal Si layer is produced by deposition, preferablysilicon oxide is formed on the surface of the single crystal Si layer,typically by thermal oxidation, before the substrates are bondedtogether. While the second substrate may preferably be a Si substratewith or without a Si oxide film formed on the surface thereof, a lighttransmitting substrate typically made of quartz or a sapphire substrate,other appropriate substrates may alternatively be used for the secondsubstrate so long as the surface to be bonded to the first substrate issufficiently flat. While FIG. 1A shows a second substrate bonded to afirst substrate with an insulation layer 15 arranged therebetween, theinsulation layer 15 may be omitted when the nonporous thin film 13 orthe second substrate is not made of Si. A thin insulating panel may bearranged between the first and second substrates when bonding themtogether.

When the nonporous thin film is made of epitaxially grown single crystalsilicon or some other similar material, the pores inside the porous Silayer can be rearranged and closed to reduce the etchability of theporous Si layer at the time of etching if it is subjected to heattreatment during the process of epitaxial growth or in a subsequentprocess. In order to avoid this problem and improve the structuralstability of the porous layer, the porous Si layer is preferablysubjected to a preliminary heat treatment operation that is conducted attemperature between 200 and 700° C. to form a thin oxide film on thewall surface of the pores (while maintaining the properties of poroussingle crystal silicon) and prevent any possible rearrangement of thepores.

A step as will be described below can be employed to produce anepitaxial silicon film that is substantially free from defects.

While a porous Si layer maintains the structure of single crystalsilicon, the epitaxial silicon film formed on the surface can showdefects attributable to the numerous pores existing on the surface ofthe porous Si layer. Therefore, it may be a good idea to hermeticallyclose the surface of the porous Si layer that is brought into contactwith the epitaxial film by means of single crystal Si.

A technique that can be used for hermetically closing the surface of theporous Si layer is a heat treatment operation to be conducted in ahydrogen-containing atmosphere. As a result of this heat treatment usinghydrogen, some of the silicon atoms on the surface of the porous Silayer will be migrated to hermetically close the pores exposed to thesurface of the porous Si layer. This heat treatment operation istypically conducted at temperature between 500 and 1,300° C., preferablybetween 900 and 1,300° C.

Apart from this technique, it may also be effective to form a siliconfilm on the surface of the porous Si layer at a very low rate to closethe pores exposed to the surface of the layer by allowing gas thatcontains silicon atoms to flow into the film forming chamber.

In the above described process of closing the pores exposed to thesurface of the porous Si layer and forming a silicon film by epitaxialgrowth after the formation of a thin oxide film on the wall surface ofthe pores, the single crystal is preferably exposed at the top of theporous Si layer to effectively close the pores. The single crystal canbe exposed by immersing the upper surface of the porous Si layer whosepores have been coated with thin oxide film in an acid such as HF toremove the thin oxide film arranged on the upper surface.

Thereafter, the entire substrate (multilayer structure) obtained bybonding the first and second substrate, the porous Si layer thereof or alayer located close to the Si layer is heated to separate the componentsubstrates along the porous Si layer by means of the generated thermalstress or by making use of the softened porous Si layer (FIG. 1B). Toachieve the separation, the entire substrate may be heated in a heattreatment furnace. Alternatively, the porous Si layer or a layer locatedclose to the porous Si layer may be locally heated by irradiating itwith a laser beam having a wavelength that can cause it to efficientlyabsorb the laser energy. Still alternatively, the porous Si layer can beheated by causing an electric current to flow along the plane of theporous Si layer or a layer located close to the porous Si layer.

Thereafter, the porous Si layer 12 is selectively removed. If thenonporous thin film is made of single crystal Si, only the porous Silayer 12 is etched off by nonelectrolytic wet chemical etching by usingan etching solution prepared for ordinary Si etching, hydrofluoric acidthat is an etching solution for selectively etching porous Si, a mixturesolution obtained by adding at least either alcohol or aqueous hydrogenperoxide to hydrofluoric acid, buffered hydrofluoric acid or a mixturesolution obtained by adding at least either alcohol or aqueous hydrogenperoxide to buffered hydrofluoric acid in order to leave on the secondsubstrate the film that has been formed on the porous layer of the firstsubstrate. As described above in detail, it is possible to selectivelyetch only the porous Si by means of an etching solution prepared forordinary Si etching because of the large surface area of the porous Silayer 12. Alternatively, the porous Si layer may be selectively removedby polishing it, using the nonporous thin film layer 13 as a polishingstopper.

When a compound semiconductor layer is formed on the porous Si layer, anetching solution that provides a high Si etching rate relative to thecompound semiconductor is used to chemically etch only the porous Silayer 12, leaving the thin single crystal compound semiconductor filmlayer 13 on the second substrate 14. Alternatively, the porous Si layer12 may be selectively removed by polishing it, using the single crystalcompound semiconductor layer 13 as a polishing stopper.

FIG. 1C shows a semiconductor article that can be produced by a methodaccording to the invention. A large nonporous thin film which istypically a single crystal Si thin film 13 is evenly and thinly formedon the entire surface of the second substrate 14. If an insulatingsubstrate is used for the second substrate 14, the preparedsemiconductor substrate can advantageously be used for producingelectronic devices that are insulated and separated from each other.

Once the residual porous Si on the first single crystal Si substrate 11is removed from the latter, the latter can be used as another firstsingle crystal Si substrate 11 or as another second substrate 14 aftersmoothing the surface if the surface has turned impermissibly coarse andsuch a smoothing operation is necessary.

Second Mode of Carrying out the Invention

FIGS. 2A through 2C illustrate a second mode of carrying out theinvention. As shown, a porous Si layer 22 and a nonporous thin film 23are formed on each of the opposite surfaces of a first single crystal Sisubstrate 21 and second substrates 24, 25 are bonded to the respectivesurfaces with an insulation layer 26 arranged between each of the secondsubstrates and the first substrate so that a pair of multilayerstructures are produced in a single process. Otherwise, themanufacturing steps of this mode are identical with those of theabove-described first mode.

Once the residual porous Si on the first single crystal Si substrate 21is removed from the latter, the latter can be used as another firstsingle crystal Si substrate 21 or as another second substrate 24 (or 25)after smoothing out the surface if the surface has turned impermissiblycoarse and such a smoothing operation is necessary.

The support substrates 24, 25 may have respective thicknesses that aredifferent from each other. The nonporous thin films 23 on the oppositesurfaces of the first substrate may be made of respective materials andhave respective thicknesses that are different from each other.

Now, the present invention will be described further by way of examples.

EXAMPLE 1

The surface layer of a first single crystal Si substrate was subjectedto anodization in a HF solution. The anodization was conducted under thefollowing conditions.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 11 (min.)

thickness of the porous Si layer: 12 (μm)

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with a thermally oxidized film of silicon.Single crystal Si was made to epitaxially grow to a thickness of 0.15 μmon the porous Si layer by means of a CVD (chemical vapor deposition)technique. This operation was conducted under the following conditions.

source gas: SiH₂ Cl₂ / H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of a Sisubstrate (second substrate) carrying a 500 nm thick SiO₂ layer thereonand prepared in advance were brought into contact with each other andput together to produce a multilayer structure.

After removing the oxide film on the rear surface of the firstsubstrate, a CO₂ laser beam was irradiated on the entire first substrateside surface of the wafer with an output power level of 500 to 1,000 W.The CO₂ laser was absorbed by the 500 nm thick SiO₂ layer arranged onthe interface of the two substrates to rapidly raise the temperature ofthe epitaxial layer and the porous Si layer that were located close toit until the two substrates were separated from each other along theunderlying porous Si layer due to the thermal stress rapidly generatedin the underlying porous Si layer. For the purpose of the invention, thelaser beam may be a continuous laser beam or a pulse laser beam.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely.

The rate of etching nonporous single crystal Si by means of the abovecited etching solution is very low and the selectivity ratio of the rateof etching porous Si relative to that of etching nonporous singlecrystal Si is as large as more than 10⁵ so that the reduction by etchingof the height of the nonporous layer (about tens of several angstroms)is practically negligible.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The thickness of the formed single crystal Silayer was observed at 100 points spreading over the entire surface ofthe substrate to find that the uniformity of the film thickness was 101nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the rootmean square of the surface roughness (Rrms) within a 50 μm square wasabout 0.2 nm, which is substantially equal to the corresponding value ofcommercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

For the purpose of comparison, a similar multilayer structure carryingno oxide film on the epitaxial Si layer was prepared to obtain the aboveidentified results.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely andthe first substrate could be used for another anodization process or foranother oxide film forming process as a second substrate.

EXAMPLE 2

The surface layer of a first single crystal Si substrate was subjectedto anodization in a HF solution. The anodization was conducted under thefollowing conditions.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 11 (min.)

thickness of the porous Si layer: 12 (μm)

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with a thermally oxidized film of silicon.P⁺ single crystal Si was made to epitaxially grow to a thickness of 0.15μm on the porous Si layer by means of a CVD (chemical vapor deposition)technique. This operation was conducted under the following conditions.B₂ H₆ was introduced as impurity gas.

source gas: SiH₂ Cl₂ /H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of a Sisubstrate (second substrate) carrying a 500 nm thick SiO₂ layer thereonand prepared in advance were brought into contact with each other andput together.

An electric current of about 10 to 100 A was made to flow only throughthe high concentration P⁺ single crystal Si layer of the first substrate(the impurity concentration of the high concentration P⁺ single crystalSi layer may be such that it can reduce the electric resistance of thelayer to allow the electric current to flow therethrough). The electriccurrent was made to flow by removing the SiO₂ to expose the highconcentration P⁺ single crystal Si layer at an end surface of the waferand pinching the wafer by means of + and - electrodes that touch onlythe end surface. As a result, the underlying porous Si layer wasabruptly subjected to thermal stress to sever the two substrates alongthe underlying porous Si layer. For the purpose of the invention, theelectric current may be a continuous current or a pulse current.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The thickness of the formed single crystal Silayer was observed at 100 points spreading over the entire surface ofthe substrate to find that the uniformity of the film thickness was 101nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

For the purpose of comparison, a similar multilayer structure carryingno oxide film on the epitaxial Si layer was prepared to obtain theabove-identified results.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely andthe first substrate could be used for another anodization process or foranother oxide film forming process as a second substrate.

EXAMPLE 3

The surface layer of a first single crystal Si substrate was subjectedto anodization in HF solution. The anodization was conducted under thefollowing conditions.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 11 (min.)

thickness of the porous Si layer: 12 (μm)

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with a thermally oxidized film of silicon.Single crystal Si was made to epitaxially grow to a thickness of 0.15 μmon the porous Si layer by means of a CVD (chemical vapor deposition)technique. This operation was conducted under the following conditions.

source gas: SiH₂ Cl₂ /H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of a Sisubstrate (second substrate) carrying a 500 nm thick SiO₂ layer thereonand prepared in advance were exposed to nitrogen plasma (in order toimprove the bonding strength) and then laid one on the other to bringthem into contact with each other. The combined substrates were thenannealed at 400° C. for 10 hours.

After removing the oxide film on the rear surface of the firstsubstrate, a CO₂ laser beam was irradiated on the entire first substrateside surface of the wafer with an output power level of 500 to 1,000 W.The CO₂ laser was absorbed by the 500 nm thick SiO₂ layer arranged onthe interface of the two substrates to rapidly raise the temperature ofthe epitaxial layer and the porous Si layer that were located close toit until the two substrates were separated from each other along theunderlying porous Si layer due to the thermal stress rapidly generatedin the underlying porous Si layer. For the purpose of the invention, thelaser beam may be a continuous laser beam or a pulse laser beam.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in an etching solution of HF/HNO₃ /CH₃ COOH type.The single crystal Si was left unetched and operated as an etchingstopper so that the porous Si was selectively etched and removedcompletely.

The rate of etching nonporous single crystal Si by means of the abovecited etching solution is very low and the reduction by etching of theheight of the nonporous layer is practically negligible.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The

thickness of the formed single crystal Si layer was observed at 100points spreading over the entire surface of the substrate to find thatthe uniformity of the film thickness was 101 nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

For the purpose of comparison, a similar multilayer structure carryingno oxide film on the epitaxial Si layer was prepared to obtain theabove-identified results.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in an HF/HNO₃ /CH₃ COOH type etching solution.The single crystal Si was left unetched and operated as etching stopperso that the porous Si was selectively etched and removed completely andthe first substrate could be used for another anodization process or foranother oxide film forming process as a second substrate.

EXAMPLE 4

The surface layer of a first single crystal Si substrate was subjectedto anodization in an HF solution. The anodization was conducted underthe following conditions.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 11 (min.)

thickness of the porous Si layer: 12 (μm)

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with a thermally oxidized film of silicon.P⁺ single crystal Si was made to epitaxially grow to a thickness of 0.15μm on the porous Si layer by means of a CVD (chemical vapor deposition)technique. This operation was conducted under the following conditions.B₂ H₆ was introduced as impurity gas.

source gas: SiH₂ Cl₂ /H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of of an Sisubstrate (second substrate) carrying a 500 nm thick SiO₂ layer thereonand prepared in advance were brought into contact with each other andput together.

An electric current of about 10 to 100 A was made to flow only throughthe high concentration P⁺ single crystal Si layer of the first substrate(the impurity concentration of the high concentration P⁺ single crystalSi layer may be such that it can reduce the electric resistance of thelayer to allow the electric current to flow therethrough). The electriccurrent was made to flow by removing the SiO₂ to expose the highconcentration P⁺ single crystal Si layer at an end surface of the waferand pinching the wafer by means of + and - electrodes that touch onlythe end surface. As a result, the underlying porous Si layer wasabruptly subjected to thermal stress to sever the two substrates alongthe underlying porous Si layer. For the purpose of the invention, theelectric current may be a continuous current or a pulse current.

Thereafter, the residual porous Si layer on the second substrate wasselectively polished. The single crystal Si was left unpolished andoperated as an etching stopper so that the porous Si was selectivelypolished and removed completely.

The rate of polishing the nonporous single crystal Si was very low andthe reduction by polishing of the height of the nonporous layer (abouttens of several angstroms) is practically negligible.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The

thickness of the formed single crystal Si layer was observed at 100points spreading over the entire surface of the substrate to find thatthe uniformity of the film thickness was 101 nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

For the purpose of comparison, a similar multilayer structure carryingno oxide film on the epitaxial Si layer was prepared to obtain the aboveidentified results.

Finally, the porous Si remaining on the first substrate was alsoselectively polished off. The single crystal Si was left unpolished andoperated as etching stopper so that the porous Si was selectivelypolished and removed completely and the first substrate could be usedfor another anodization process or for another oxide film formingprocess as a second substrate.

EXAMPLE 5

The surface layer of a first single crystal Si substrate was subjectedto anodization in a HF solution. The anodization was conducted under thefollowing conditions.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 11 (min.)

thickness of the porous Si layer: 12 (μm)

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with a thermally oxidized film of silicon.P⁺ single crystal Si was made to epitaxially grow to a thickness of 0.15μm on the porous Si layer by means of a CVD (chemical vapor deposition)technique. This operation was conducted under the following conditions.B₂ H₆ was introduced as impurity gas.

source gas: SiH₂ Cl₂ /H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of a quartzsubstrate (second substrate) prepared in advance were exposed tonitrogen plasma and then put together to bring them into contact witheach other. The combined substrates were then annealed at 200° C. for 10hours.

An electric current of about 10 to 100 A was made to flow only throughthe high concentration P⁺ single crystal Si layer of the first substrate(the impurity concentration of the high concentration P⁺ single crystalSi layer may be such that it can reduce the electric resistance of thelayer to allow the electric current to flow therethrough). The electriccurrent was made to flow by removing the SiO₂ to expose the highconcentration P⁺ single crystal Si layer at an end surface of the waferand pinching the wafer by means of + and - electrodes that touch onlythe end surface. As a result, the underlying porous Si layer wasabruptly subjected to thermal stress to sever the two substrates alongthe underlying porous Si layer. For the purpose of the invention, theelectric current may be a continuous current or a pulse current.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as etching stopper sothat the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The thickness of the formed single crystal Silayer was observed at 100 points spreading over the entire surface ofthe substrate to find that the uniformity of the film thickness was 101nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

For the purpose of comparison, a similar multilayer structure carryingno oxide film on the epitaxial Si layer was prepared to obtain theabove-identified results.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely andthe first substrate could be used for another anodization process.

EXAMPLE 6

The surface layer of a first single crystal Si substrate was subjectedto anodization in a HF solution. The anodization was conducted under thefollowing conditions.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 11 (min.)

thickness of the porous Si layer: 12 (μm)

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with thermally oxidized film of silicon.Single crystal GaAs was made to epitaxially grow to a thickness of 1 μmon the porous Si layer by means of a CVD (chemical vapor deposition)technique. This operation was conducted under the following conditions.

source gas: TMG/AsH₃ /H₂

gas pressure: 80 Torr

temperature: 700° C.

The surface of the GaAs layer and the corresponding surface of a Sisubstrate (second substrate) prepared in advance were brought intocontact with each other and put together.

After removing the oxide film on the rear surface of the firstsubstrate, a CO₂ laser beam was irradiated on the entire first substrateside surface of the wafer with an output power level of 500 to 1,000 W.The CO₂ laser was absorbed by the GaAs layer to rapidly raise thetemperature of the nearby porous Si layer until the two substrate wereseparated from each other along the underlying porous Si layer due tothe thermal stress rapidly generated in the underlying porous Si layer.For the purpose of the invention, the laser beam may be a continuouslaser or a pulse laser beam.

Thereafter, the residual porous Si layer on the second substrate wasetched off by means of:

ethylenediamine+pyrocatechol+water (at a ratio of 17 ml:3 g:8 ml) at110° C.

The single crystal GaAs was left unetched and operated as an etchingstopper so that the porous Si and the oxidized porous Si wereselectively etched and removed completely.

The rate of etching nonporous single crystal GaAs by means of the abovecited etching solution is very low and the reduction by etching of theheight of the nonporous layer (about tens of several angstroms) ispractically negligible.

Thus, a single crystal GaAs layer was formed to a thickness of 1 μm onsilicon. The thickness of the formed single crystal GaAs layer wasobserved at 100 points spreading over the entire surface of thesubstrate to find that the degree of uniformity of the film thicknesswas 1 μm±29.8 nm.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the GaAs layer and an excellent degree of crystallinityhad been maintained.

For the purpose of comparison, a GaAs layer was formed on an insulationfilm by using a Si substrate carrying thereon an oxide film as a supportsubstrate to obtain the above-identified results.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely andthe first substrate could be used for another anodization process or foranother bonding process as a second substrate.

EXAMPLE 7

The surface layer of a first single crystal Si substrate was subjectedto anodization in a HF solution. The anodization was conducted under thefollowing conditions.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 11 (min.)

thickness of the porous Si layer: 12 (μm)

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with thermally oxidized film. Singlecrystal InP was made to epitaxially grow to a thickness of 1 μm on theporous Si layer by means of a MOCVD (metal organic chemical vapordeposition) technique.

The surface of the InP layer and that of a quartz substrate (secondsubstrate) prepared in advance were brought into contact with each otherand put together. The combined substrates were annealed at 200° C. for10 hours.

After removing the oxide film on the rear surface of the firstsubstrate, a CO₂ laser beam was irradiated on the entire first substrateside surface of the wafer with an output power level of 500 to 1,000 W.The CO₂ laser was absorbed by the InP layer to rapidly raise thetemperature of the nearby porous Si layer until the two substrates wereseparated from each other along the underlying porous Si layer due tothe thermal stress rapidly generated in the underlying porous Si layer.For the purpose of the invention, the laser beam may be a continuouslaser beam or a pulse laser beam.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal InP was left unetched and operated as etching stopper sothat the porous Si was selectively etched and removed completely.

Thus, a single crystal InP layer was formed to a thickness of 1 μm onthe quartz substrate. The

thickness of the formed single crystal InP layer was observed at 100points spreading over the entire surface of the substrate to find thatthe degree of uniformity of the film thickness was 1 μm±29.0 nm.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the InP layer and an excellent degree of crystallinity hadbeen maintained.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated an etching stopper sothat the porous Si was selectively etched and removed completely and thefirst substrate could be used for another anodization process.

EXAMPLE 8

The surface of a first single crystal Si substrate was subjected toanodization in a HF solution on the two opposite sides thereof. Theanodization was conducted under the following conditions for 11 minutesfor each side.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 11×2 (min.)

thickness of the porous Si layer: 12 (μm)

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with thermally oxidized film of silicon.Single crystal Si was made to epitaxially grow to a thickness of 0.15 μmon each of the oppositely disposed porous Si layers by means of a CVD(chemical vapor deposition) technique. This operation was conductedunder the following conditions.

source gas: SiH₂ Cl₂ /H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of each of the epitaxially grown Silayers.

The oppositely disposed surfaces of the SiO₂ layer and the correspondingsurfaces of a pair of Si substrates (second substrates), each carrying a500 nm thick SiO₂ layer thereon and prepared in advance, were broughtinto contact with each other and put together.

After removing the oxide film on the rear surface of each of the secondsubstrates, a CO₂ laser beam was irradiated on the entire surface ofeach of the second substrates of the wafer with an output power level of500 to 1,000 W. The CO₂ laser was absorbed by the 500 nm thick SiO₂layer arranged on the interface of the substrates on each side of thewafer to rapidly raise the temperature of the epitaxial layer and theporous Si layer that were located close to it, until the substrates wereseparated from each other along the underlying porous Si layer due tothe thermal stress rapidly generated in the underlying porous Si layer.For the purpose of the invention, the laser beam may be a continuouslaser beam or a pulse laser beam.

Thereafter, the residual porous Si layer on each of the secondsubstrates was selectively etched off in a mixture solution of 49%hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring thesolution constantly. The single crystal Si was left unetched andoperated as an etching stopper so that the porous Si was selectivelyetched and removed completely.

Thus, a pair of single crystal Si layers were formed to a thickness of0.1 μm on the respective silicon oxide films. The thickness of each ofthe formed single crystal Si layers was observed at 100 points spreadingover the entire surface of the substrate to find that the uniformity ofthe film thickness was 101 nm±3 nm.

Then, the wafer was subjected to a heat treatment operation at 1,100° C.for an hour in a hydrogen atmosphere. The surface coarseness wasobserved by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layers and an excellent degree of crystallinity hadbeen maintained.

For the purpose of comparison, a similar multilayer structure carryingno oxide film on the epitaxial Si layer was prepared to obtain the aboveidentified results.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely andthe first substrate could be used for another anodization process or foranother oxide film forming process as a second substrate.

EXAMPLE 9

A SiO₂ layer was formed on the surface of a first single crystal Sisubstrate to a thickness of 100 nm by thermal oxidation. Hydrogen ionswere implanted into the principal surface of the substrate to aconcentration of 1×10¹⁷ /cm² with an acceleration voltage of 25 keV. Asa result, a porous structure due to hydrogen bubbles was formed withinthe substrate and centered at a depth of 0.3 μm from the surface.

The surface of the SiO₂ layer of the first Si substrate and thecorresponding surface of another Si substrate (second substrate)carrying a 500 nm thick SiO₂ layer thereon and prepared in advance werebrought into contact with each other and put together.

After removing the oxide film on the rear surface of the firstsubstrate, a CO₂ laser beam was irradiated on the entire first substrateside surface of the wafer with an output power level of 500 to 1,000 W.The CO₂ laser was absorbed by the 500 nm thick SiO₂ layer arranged onthe interface of the two substrates to rapidly raise the temperature ofthe epitaxial layer and the porous Si layer that were located close toit until the two substrates were separated from each other along theunderlying porous Si layer due to the thermal stress rapidly generatedin the underlying porous Si layer. For the purpose of the invention, thelaser beam may be a continuous laser beam or a pulse laser beam.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The thickness of the formed single crystal Silayer was observed at 100 points spreading over the entire surface ofthe substrate to find that the uniformity of the film thickness was 101nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

For the purpose of comparison, a similar multilayer structure carryingno oxide film on the epitaxial Si layer was prepared to obtain the aboveidentified results.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely. Thefirst substrate could be used for another anodization process or foranother oxide film forming process as a second substrate.

EXAMPLE 10

The surface layer of a first single crystal Si substrate was subjectedto anodization in a HF solution. The anodization was conducted under thefollowing conditions.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ : C₂ H₅ OH=1:1:1

duration: 11 (min.)

thickness of the porous Si layer: 12 (μm)

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with a thermally oxidized film of silicon.Single crystal Si was made to epitaxially grow to a thickness of 0.15 μmon the porous Si layer by means of a CVD (chemical vapor deposition)technique. This operation was conducted under the following conditions.

source gas: SiH₂ Cl₂ /H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of the epitaxially grown Si layer.

Hydrogen ions were implanted into the principal surface of the substrateto a concentration of 5×10¹⁶ /cm² with an acceleration voltage of 180keV.

The surface of the SiO₂ layer and the corresponding surface of an Sisubstrate (second substrate) carrying a 500 nm thick SiO₂ layer thereonand prepared in advance were brought into contact with each other andput together.

After removing the oxide film on the rear surface of the firstsubstrate, a CO₂ laser beam was irradiated on the entire first substrateside surface of the wafer with an output power level of 500 to 1,000 W.The CO₂ laser was absorbed by the 500 nm thick SiO₂ layer arranged onthe interface of the two substrates to rapidly raise the temperature ofthe epitaxial layer and the porous Si layer that were located close toit, until the two substrates were separated from each other along theunderlying porous Si layer due to the thermal stress rapidly generatedin the underlying porous Si layer. For the purpose of the invention, thelaser beam may be a continuous laser beam or a pulse laser beam. Thesite of separation could be substantially rigorously controlled due tothe ion implantation and the substrates were separated along a depth ofabout 1.5 μm of the porous Si layer from the bonded surface of the SiO₂.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as etching stopper sothat the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The thickness of the formed single crystal Silayer was observed at 100 points spreading over the entire surface ofthe substrate to find that the uniformity of the film thickness was 101nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

For the purpose of comparison, a similar multilayer structure carryingno oxide film on the epitaxial Si layer was prepared to obtain theabove-identified results.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely. Thefirst substrate could be used for another anodization process or foranother oxide film forming process as a second substrate.

EXAMPLE 11

The surface layer of a first single crystal Si substrate was subjectedto anodization in an a HF solution. The anodization was conducted underthe following conditions.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 11 (min.)

thickness of the porous Si layer: 12 (μm)

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with a thermally oxidized film of silicon.Single crystal Si was made to epitaxially grow to a thickness of 0.15 μmon the porous Si layer by means of a CVD (chemical vapor deposition)technique. This operation was conducted under the following conditions.

source gas: SiH₂ Cl₂ /H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Sisubstrate (second substrate) were brought into contact with each otherand put together.

After removing the oxide film on the rear surface of the firstsubstrate, a CO₂ laser beam was irradiated on the entire first substrateside surface of the wafer with an output power level of 500 to 1,000 W.The CO₂ laser was absorbed by the 500 nm thick SiO₂ layer arranged onthe interface of the two substrates to rapidly raise the temperature ofthe epitaxial layer and the porous Si layer that were located close toit, until the two substrates were separated from each other along theunderlying porous Si layer due to the thermal stress rapidly generatedin the underlying porous Si layer. For the purpose of the invention, thelaser beam may be a continuous laser beam or a pulse laser beam.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as etching stopper sothat the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The thickness of the formed single crystal Silayer was observed at 100 points spreading over the entire surface ofthe substrate to find that the uniformity of the film thickness was 101nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely. Thefirst substrate could be used for another anodization process or foranother bonding process as a second substrate.

EXAMPLE 12

The surface layer of a first single crystal Si substrate was subjectedto anodization in an HF solution. The anodization was conducted underthe following conditions.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 5.5 (min.)

thickness of the porous Si layer: 6 (μm) and subsequently

current density: 70 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 0.5 (min.)

thickness of the porous Si layer: 5 (μm)

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with thermally oxidized film of silicon.Single crystal Si was made to epitaxially grow to a thickness of 0.15 μmon the porous Si layer by means of a CVD (chemical vapor deposition)technique. This operation was conducted under the following conditions.

source gas: SiH₂ Cl₂ /H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of a Sisubstrate (second substrate) were brought into contact with each otherand put together.

After removing the oxide film on the rear surface of the firstsubstrate, a CO₂ laser beam was irradiated on the entire first substrateside surface of the wafer with an output power level of 500 to 1,000 W.The CO₂ laser was absorbed by the 500 nm thick SiO₂ layer arranged onthe interface of the two substrates to rapidly raise the temperature ofthe epitaxial layer and the porous Si layer that were located close toit until the two substrates were separated from each other along theunderlying porous Si layer due to the thermal stress rapidly generatedin the underlying porous Si layer. For the purpose of the invention, thelaser beam may be a continuous laser beam or a pulse laser beam.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The thickness of the formed single crystal Silayer was observed at 100 points spreading over the entire surface ofthe substrate to find that the uniformity of the film thickness was 101nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely andthe first substrate could be used for another anodization process or foranother bonding process as a second substrate.

EXAMPLE 13

The surface layer of a first single crystal Si substrate was subjectedto anodization in a HF solution. The anodization was conducted under thefollowing conditions.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 3.5 (min.)

thickness of the porous Si layer: 4 (μm ),

subsequently

current density: 100 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 0.2 (min.)

thickness of the porous Si layer: 3 (μm)

and then

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 3.5 (min.)

thickness of the porous Si layer: 4 (μm).

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with a thermally oxidized film of silicon.Single crystal Si was made to epitaxially grow to a thickness of 0.15 μmon the porous Si layer by means of a CVD (chemical vapor deposition)technique. This operation was conducted under the following conditions.

source gas: SiH₂ Cl₂ /H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of a Sisubstrate (second substrate) were brought into contact with each otherand put together.

After removing the oxide film on the rear surface of the firstsubstrate, a CO₂ laser beam was irradiated on the entire first substrateside surface of the wafer with an output power level of 500 to 1,000 W.The CO₂ laser was absorbed by the 500 nm thick SiO₂ layer arranged onthe interface of the two substrates to rapidly raise the temperature ofthe epitaxial layer and the porous Si layer that were located close toit, until the two substrates were separated from each other along theunderlying porous Si layer due to the thermal stress rapidly generatedin the underlying porous Si layer. For the purpose of the invention, thelaser beam may be a continuous laser beam or a pulse laser beam.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The thickness of the formed single crystal Silayer was observed at 100 points spreading over the entire surface ofthe substrate to find that the uniformity of the film thickness was 101nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely. Thefirst substrate could be used for another anodization process or foranother bonding process as a second substrate.

EXAMPLE 14

The surface layer of a first single crystal Si substrate was subjectedto anodization in a HF solution. The anodization was conducted under thefollowing conditions.

current density: 7 (mA.cmM⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 11 (min.)

thickness of the porous Si layer: 12 (μm)

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with thermally oxidized film of silicon.Single crystal Si was made to epitaxially grow to a thickness of 0.15 μmon the porous Si layer by means of a CVD (chemical vapor deposition)technique. This operation was conducted under the following conditions.

source gas: SiH₂ Cl₂ /H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Sisubstrate (second substrate) were brought into contact with each otherand put together for bonding.

Then, the bonded substrate were heated to about 1,250° C. in a heattreatment furnace. They were separated from each other along theunderlying porous Si layer due to the thermal stress rapidly generatedin the porous Si layer. For the purpose of the invention, the substratesmay be subjected to another heat treatment to improve their bondingstrength.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The thickness of the formed single crystal Silayer was observed at 100 points spreading over the entire surface ofthe substrate to find that the uniformity of the film thickness was 101nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

For the purpose of comparison, a similar multilayer structure carryingno oxide film on the epitaxial Si layer was prepared to obtain theabove-identified results.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely andthe first substrate could be used for another anodization process or foranother bonding process as a second substrate.

EXAMPLE 15

The surface layer of a first single crystal Si substrate was subjectedto anodization in a HF solution. The anodization was conducted under thefollowing conditions.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 11 (min.)

thickness of the porous Si layer: 12 (μm)

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with thermally oxidized film of silicon.Single crystal Si was made to epitaxially grow to a thickness of 0.15 μmon the porous Si layer by means of a CVD (chemical vapor deposition)technique. This operation was conducted under the following conditions.

source gas: SiH₂ Cl₂ /H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Sisubstrate (second substrate) carrying a 500 nm thick SiO₂ layer thereonand prepared in advance were brought into contact with each other andput together for bonding.

Then, the bonded substrates were heated to about 1,250° C. in a heattreatment furnace. They were separated from each other along theunderlying porous Si layer due to the thermal stress rapidly generatedin the porous Si layer. For the purpose of the invention, the substratesmay be subjected to another heat treatment to improve their bondingstrength.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The thickness of the formed single crystal Silayer was observed at 100 points spreading over the entire surface ofthe substrate to find that the uniformity of the film thickness was 101nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

For the purpose of comparison, a similar multilayer structure carryingno oxide film on the epitaxial Si layer was prepared to obtain theabove-identified results.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely andthe first substrate could be used for another anodization process or foranother oxide film forming process as a second substrate.

EXAMPLE 16

The surface layer of a first single crystal Si substrate was subjectedto anodization in a HF solution. The anodization was conducted under thefollowing conditions.

current density: 7 (mA.cM⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 3.5 (min.)

thickness of the porous Si layer: 4 (μm),

subsequently

current density: 100 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 0.2 (min.)

thickness of the porous Si layer: 3 (μm)

and then

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 3.5 (min.)

thickness of the porous Si layer: 4 (μm). T

he substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with a thermally oxidized film of silicon.Single crystal Si was made to epitaxially grow to a thickness of 0.15 μmon the porous Si layer by means of a CVD (chemical vapor deposition)technique. This operation was conducted under the following conditions.

source gas: SiH₂ Cl₂ /H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of a Sisubstrate (second substrate) were brought into contact with each otherand put together for bonding.

Then, the bonded substrates were heated to about 600 to 1,200° C. in aheat treatment furnace. They were separated from each other along theporous Si layer due to the thermal stress rapidly generated in theporous Si layer. For the purpose of the invention, the substrates may besubjected to another heat treatment to improve their bonding strength.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The thickness of the formed single crystal Silayer was observed at 100 points spreading over the entire surface ofthe substrate to find that the uniformity of the film thickness was 101nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely andthe first substrate could be used for another anodization process or foranother bonding process as a second substrate.

EXAMPLE 17

The surface layer of a first single crystal Si substrate was subjectedto anodization in a HF solution. The anodization was conducted under thefollowing conditions.

current density: 7 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 5.5 (min.)

thickness of the porous Si layer: 6 (μm),

and subsequently

current density: 70 (mA.cm⁻²)

anodization solution: HF:H₂ O:C₂ H₅ OH=1:1:1

duration: 0.5 (min.)

thickness of the porous Si layer: 5 (μm).

The substrate was then oxidized at 400° C. for an hour in an oxygenatmosphere. As a result of the oxidation, the wall surfaces of the poresof the porous Si were covered with thin thermally oxidized film ofsilicon.

Thereafter, the thin oxide film produced on the uppermost surface of thesubstrate where the porous layer had been formed was removed byimmersing it in a 1.25% HF solution. Subsequently, the resultingsubstrate was subjected to heat treatment at 1,050° C. and 760 Torr for1 minute in a flow of H₂ flowing at a rate of 230 l/min. and for more 5minutes after adding SiH₄ by 50 sccm.

Then, single crystal Si was made to epitaxially grow to a thickness of0.15 μm on the porous Si layer by means of a CVD (chemical vapordeposition) technique. This operation was conducted under the followingconditions.

source gas: SiH₂ Cl₂ /H₂

gas flow rate: 0.5/180 l/min.

gas pressure: 80 Torr

temperature: 950° C.

growth rate: 0.3 μm/min.

Additionally, a SiO₂ layer was formed to a thickness of 100 nm bythermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of a Sisubstrate (second substrate) were brought into contact with each otherand put together for bonding.

After removing the oxide film on the rear surface of the firstsubstrate, a CO₂ laser beam was irradiated on the entire first substrateside surface of the wafer with an output power level of 500 to 1,000 W.The CO₂ laser was absorbed by the 500 nm thick SiO₂ layer arranged onthe interface of the two substrates to rapidly raise the temperature ofthe epitaxial layer and the porous Si layer that were located close toit until the two substrates were separated from each other along theunderlying porous Si layer due to the thermal stress rapidly generatedin the underlying porous Si layer. For the purpose of the invention, thelaser beam may be a continuous laser beam or a pulse laser beam.

Thereafter, the residual porous Si layer on the second substrate wasselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1 μm onthe silicon oxide film. The

thickness of the formed single crystal Si layer was observed at 100points spreading over the entire surface of the substrate to find thatthe uniformity of the film thickness was 101 nm±3 nm.

Then, the substrate was subjected to a heat treatment operation at1,100° C. for an hour in a hydrogen atmosphere. The surface coarsenesswas observed by means of atomic force microscopy to find that the Rrmswithin a 50 μm square was about 0.2 nm, which is substantially equal tothe corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electronmicroscope, it was confirmed that no new crystal defects had beenintroduced in the Si layer and an excellent degree of crystallinity hadbeen maintained.

Finally, the porous Si remaining on the first substrate was alsoselectively etched off in a mixture solution of 49% hydrofluoric acidand 30% aqueous hydrogen peroxide, stirring the solution constantly. Thesingle crystal Si was left unetched and operated as an etching stopperso that the porous Si was selectively etched and removed completely andthe first substrate could be used for another anodization process or foranother bonding process as a second substrate.

What is claimed is:
 1. A method of manufacturing a semiconductor article, characterized by comprising steps of:preparing a first substrate including a silicon substrate having a porous silicon layer and a nonporous semiconductor layer arranged on said porous silicon layer; bonding said first substrate and a second substrate to produce a multilayer structure with said nonporous semiconductor layer located inside; separating said multilayer structure along said porous silicon layer by causing an electric current to flow through said porous silicon layer for heating said multilayer structure, so that said nonporous semiconductor layer is arranged on said separated second substrate.
 2. A method of manufacturing a semiconductor article, characterized by comprising steps of:preparing a first substrate including a silicon substrate having a porous silicon layer and a nonporous semiconductor layer arranged on said porous silicon layer; bonding said first substrate and a second substrate to produce a multilayer structure with said nonporous semiconductor layer located inside; separating said multilayer structure along said porous silicaon layer by locally heating said porous silicon layer and its proximity, so that said nonporous semiconductor layer is arranged on said separated second substrate.
 3. A method of manufacturing a semiconductor article, characterized by comprising steps of:preparing a first substrate including a silicon substrate having a porous silicon layer and a nonporous semiconductor layer arranged on said porous silicon layer; bonding said first substrate and a second substrate to produce a multilayer structure with said nonporous semiconductor layer located inside; separating said multilayer structure along said porous silicon layer by laser irradiation for heating said multilayer structure, so that said nonporous semiconductor layer is arranged on said separated second substrate.
 4. A method of manufacturing a semiconductor article according to claims 1 or 2, wherein said heating is an operation of partly heating said multilayer structure.
 5. A method of manufacturing a semiconductor article according to claim 4, wherein said laser is carbon dioxide laser.
 6. A method of manufacturing a semiconductor article according to claim 5, wherein said laser is carbon dioxide laser.
 7. A method of manufacturing a semiconductor article according to claim 4, wherein said heating is realized by causing an electric current to flow through said porous silicon layer.
 8. A method of manufacturing a semiconductor article according to claims 1 or 2, wherein a porous silicon layer is formed on the two surfaces of said silicon substrate and thereafter a nonporous semiconductor layer is formed on the two porous silicon layers.
 9. A method of manufacturing a semiconductor article according to claims 1 or 2, wherein said porous silicon layer is obtained through anodization of said silicon substrate.
 10. A method of manufacturing a semiconductor article according to claims 1, 2, or 3, wherein said first substrate is made to carry thereon said porous silicon layer formed by implanting ions of an element selected from a rare gas element, hydrogen and nitrogen into said silicon substrate into an ion implantation region of said silicon substrate having a depth from the surface of said substrate and comprises a surface layer constituted by said nonporous semiconductor layer.
 11. A method of manufacturing a semiconductor article according to claims 1, 2, or 3, wherein said first substrate is formed by forming said porous silicon layer in said silicon substrate and thereafter forming said nonporous semiconductor layer on said porous silicon layer.
 12. A method of manufacturing a semiconductor article according to claims 1, 2, or 3, wherein said nonporous semiconductor layer is constituted by a single crystal silicon layer.
 13. A method of manufacturing a semiconductor article according to claim 12, wherein said single crystal silicon layer is formed by epitaxial growth.
 14. A method of manufacturing a semiconductor article according to claim 12, wherein said first substrate comprises a silicon oxide layer formed on the surface of said single crystal silicon layer.
 15. A method of manufacturing a semiconductor article according to claim 14, wherein said silicon oxide layer is formed by thermal oxidation.
 16. A method of manufacturing a semiconductor article according to claims 1, 2, or 3, wherein said nonporous semiconductor layer comprises a compound semiconductor layer.
 17. A method of manufacturing a semiconductor article according to claim 16, wherein said compound semiconductor layer has a single crystal structure.
 18. A method of manufacturing a semiconductor article according to claims 1, 2 or 3, wherein said second substrate is a single crystal silicon substrate.
 19. A method of manufacturing a semiconductor article according to claims 1, 2 or 3, wherein said second substrate is prepared by forming an oxide film on a single crystal silicon substrate.
 20. A method of manufacturing a semiconductor article according to claims 1, 2 or 3, wherein said second substrate is a light transmitting substrate.
 21. A method of manufacturing a semiconductor article according to claim 20, wherein said light transmitting substrate is a glass substrate.
 22. A method of manufacturing a semiconductor article according to claims 1, 2 or 3, wherein said bonding step is conducted by bringing said two substrate into close contact with each other.
 23. A method of manufacturing a semiconductor article according to claims 1, 2 or 3, wherein said step is conducted through anodic bonding, pressurization or heat treatment.
 24. A method of manufacturing a semiconductor article according to claims 1, 2 or 3, wherein said step of removing said porous silicon layer is performed by polishing said substrate.
 25. A method of manufacturing a semiconductor article according to claims 1, 2 or 3, wherein said step of removing said porous silicon layer is performed by etching said substrate.
 26. A method of manufacturing a semiconductor article according to claim 25, wherein said etching is conducted by using hydrofluoric acid.
 27. A method of separating a semiconductor layer, comprising steps of:preparing a first substrate including a silicon substrate having a porous silicon layer and a nonporous semiconductor layer arranged on said porous silicon layer; bonding said first substrate and a second substrate to produce a multilayer structure with said nonporous semiconductor layer located inside; separating said nonporous semiconductor layer from said first substrate along said porous silicon layer by laser irradiation for heating said multilayer structure.
 28. A method of separating a semiconductor layer, comprising steps of:preparing a first substrate including a silicon substrate having a porous silicon layer and a nonporous semiconductor layer arranged on said porous silicon layer; bonding said first substrate and a second substrate to produce a multilayer structure with said nonporous semiconductor layer located inside; separating said nonporous semiconductor layer from said first substrate along said porous silicon layer by causing an electric current to flow through said porous silicon layer for heating said multilayer structure.
 29. A method of separating a semiconductor layer, comprising steps of:preparing a first substrate including a silicon substrate having a porous silicon layer and a nonporous semiconductor layer arranged on said porous silicon layer; bonding said first substrate and a second substrate to produce a multilayer located inside; separating said nonporous semiconductor layer from said first substrate along said porous silicon layer by locally heating said porous silicon layer and its proximity. 